Currently, commercial photoactivated dental restorative resins are based on dimethacrylates and the reaction mechanism is through chain-growth free radical polymerization. Existing dimethacrylate systems are popular for fillings and other dental prostheses because of their esthetic merit and “cure-on-command” feature.
Such dental restorative materials are often mixed with 45 to 85% by weight (wt %) silanized filler compounds such as barium, strontium, zirconia silicate and/or amorphous silica to match the color and opacity to a particular use or tooth. The filler is typically in the form of particles with a size ranging from 0.01 to 5.0 micrometers.
The photoactivated restorative materials are often sold in separate syringes or single-dose capsules of different shades. If provided in a syringe, the user dispenses (by pressing a plunger or turning a screw adapted plunger on the syringe) the necessary amount of restorative material from the syringe onto a suitable mixing surface. Then the material is placed directly into the cavity, mold, or location of use. If provided as a single-dose capsule, the capsule is placed into a dispensing device that can dispense the material directly into the cavity, mold, etc. After the restorative material is placed, it is photopolymerized or cured by exposing the restorative material to the appropriate light source. The resulting cured polymer may then be finished or polished as necessary with appropriate tools. Such dental restoratives can be used for direct anterior and posterior restorations, core build-ups, splinting and indirect restorations including inlays, onlays and veneers.
Although easy to use, these systems have several drawbacks, primarily associated with the polymerization volume shrinkage and shrinkage stress, and poor conversion of the dimethacrylate systems' monomers into polymer. The current systems can only reach a final double bond conversion of 55 to 75%, which not only contributes to the insufficient wear resistance and mechanical properties, but also jeopardizes the biocompatibility of the composites due to the leachable unreacted monomers. Dimethacrylate based resins exhibit significant volumetric shrinkage during polymerization and the induced shrinkage stress results in tooth-composite adhesive failure, initiating microleakage and recurrent caries, which significantly reduces the longevity and utility of current dental restorative composite. Furthermore, as one tries to increase the final double bond conversion to reduce the unreacted monomers, the volumetric shrinkage and shrinkage stress unfortunately also increase, which has been a persisting problem since the development of this class of resins.
Yet another drawback associated with the current systems is odor. Because the polymers are polymerized in the mouth, odors are highly perceptible by the patient. Dimethacrylate based systems, due to the low conversion, after polymerization experience a high amount of leaching of unreacted monomers, which results in an unwanted and offensive taste and odor for some period of time after the procedure.